Voice is stored and processed in the form of voltage signals in audio information processing systems. As the volume of the voice changes greatly, the voltage signal which reflects the volume of voice also changes greatly and may be clamped within the system circuitry or by power supplies with a fixed gain amplifier. Therefore the clamped audio signal may have serious distortion and may damage external equipment. To solve such issues, the gain control system, particularly the automatic gain control system, is widely used in audio amplification systems.
FIG. 1 is a single-ended input and single-ended output gain control unit 10. VIN is an input signal and VO is an output signal of the gain control unit 10. The positive input end of the amplifier U1 is coupled to a bias voltage which may be half of the power supply voltage (VDD/2). For better understanding, the bias voltage is at a level equal to VDD/2 in the following analysis. The negative input end of the amplifier U1 is coupled to a resistor network through switches S1, S2, S3 and S4. The output end of the amplifier U1 is coupled to VO.
Switches S1, S2, S3 and S4 are controlled by a 2-bit digital signal. Variety of gains will be obtained when the switches are in different ON (closed) or OFF (opened) states. This gain control unit is simple and practical, but may induce a glitch on the output signal when the gain of the gain control unit is shifted.
FIG. 2A depicts a wave form of an input signal 201 according to the gain control units 10 shown in FIG. 1. FIG. 2B depicts a wave form of an output signal 202 corresponding to the input signal in FIG. 2A. At the moment T1, the gain of the amplifier is changed. A dotted line 203 reflects the wave form of the output signal if the gain is not changed. The output signal drops from dotted line 203 and a serious glitch is generated at time T1 when the digital control signal is changed. The glitch causes a drastic current change which may damage external devices.